The present invention relates to a fuel injection system for externally ignited internal combustion engines. The system comprises an air sensing or measuring device comprising a valve which is exposed to the air stream in the suction tube of the engine. The rotation of this valve by the air against a restoring force constitutes a gauge for the amount of air flowing through the suction tube.
An air measuring device of this type is designed to measure as accurately as possible the amount of air flowing through the suction tube in order to meter a corresponding quantity of fuel to the quantity of air. To enable simple air measuring means to be used and to enable the quantity of air and fuel to be proportioned through influencing the fuel injection system while avoiding the need to make subsequent major adjustments, the relationships should be linear, for example, by maintaining two opposed non-linear but similar functions.
In the case of a known air measuring device of the above type, a baffle plate, which is vertically exposed to the air stream, is pushed according to the impedance type flow valve principle (hereinafter impedance principle) by an impedance induced force against a constant restoring force such that there is a linear relationship between the displacement path and the air flowing through the suction tube. Although the constant charging deficiency caused by the impedance is not disadvantageous in the case of small quantities of air, i.e. small air flow, at full load the charging deficiency in the engine cylinders is undesirable.
In the case of another known air measuring device operating according to the impedance principle the air measuring valve is disposed on one side and is swivelled about a pivot axis according to the quantity of air flowing through the suction tube. As a result of the changed angle of attack on the leading face of the air measuring device in the air flow direction and the associated variation in the differential pressure, the positioning force on the air measuring device decreases as the opening angle increases. This has the advantage that at full load reduced charging deficiencies occur. However, it has the disadvantage that at full load measuring becomes relatively inaccurate and the restoring force is not constant, with the resulting disadvantages of influencing the fuel injection system with non-linear proportioning of the air quantity and fuel quantity.
In the case of another known air measuring device, the valve is eccentrically mounted such that a smaller part of the valve projects into the portion of the suction tube in front of the mounting and the other part of the valve projects behind the mounting. With this air measuring device, the impedance, that is, the difference in the pressure in front of and behind the valve, is initially effective, i.e. when the valve is closed, to move the valve in its opening direction and subsequently and in accordance with the airfoil principle, the lift acting on the valve is also effective. Essentially only the differential surface between the larger and smaller surface parts of the valve acts as the resulting effective surface of the valve. The force engaging this effective surface is derived from the effective surface area times the associated air index (times ram pressure) acting on this effective surface. This force acts either as a force directed at right angles to the flow direction, or as an impedance induced force acting in the flow direction. This force forms with respect to the axis of rotation of the valve, the torque (adjusting moment) acting on the valve. With a sudden transition, in the case of this known air measuring device from the impedance principle to the combined impedance-airfoil principle, virtually undeterminable functions occur between the air quantity and the regulating distance of the valve and to ensure reliable fuel metering these must be corrected, either in the fuel metering device per se or by adjusting the restoring force acting on the valve.